Description: mm: thp: make the THP mapcount atomic against __split_huge_pmd_locked()
References:
 https://bugs.chromium.org/p/project-zero/issues/detail?id=2045
Notes:
 carnil> Another 4.9-stable-only commit is needed as per
 carnil> https://lore.kernel.org/stable/20210226162200.20548-1-vbabka@suse.cz/
 carnil> The followup commit was commited as cc916628eae7 ("mm, thp:
 carnil> make do_huge_pmd_wp_page() lock page for testing mapcount") in
 carnil> 4.9.259.
Bugs:
upstream: released (5.8-rc1) [c444eb564fb16645c172d550359cb3d75fe8a040]
4.19-upstream-stable: released (4.19.129) [453d8a481b127edfa7fb76af1bc7586b7a63bdd2]
4.9-upstream-stable: released (4.9.228) [d93d7bd61072a03dde173d36ae5815488a585fc0]
sid: released (5.7.6-1)
4.19-buster-security: released (4.19.131-1)
4.9-stretch-security: released (4.9.228-1)